As in the case of a motor-vehicle, the motorcycle attitude will vary with its load, the longitudinal profile of the road, the torque exerted on the driving wheel, braking and acceleration forces, effects due to aerodynamic forces, and the compression of the tyres. Other phenomena, e.g. relatively low-frequency pitching movements, may also result in variation of the attitude of a motorcycle. It should be noted that mean attitude variations increase in inverse proportion to the size of the wheel-base of the vehicle, and consequently these attitude variations are much greater in the case of a motorcycle than in the case of a conventional motor vehicle.
A number of automatic correction devices have already been previously proposed for motor vehicles, such as touring cars, trucks or the like. Devices of this kind are described, inter alia, in French Patents granted under the Nos. 1,358,632, 1,409,173, 1,418,215, 1,419,822, 1,438,576, 1,458,985, 1,475,011, and its First Certificate of Addition No. 93,855. All these various automatic correction devices for the vertical orientation of motor vehicle headlamps comprise means adapted to detect variations in the relative positions between the wheels and the vehicle chassis, and means for transmitting such information to a device for controlling the vertical orientation of each headlamp, such device acting on the latter through the agency of a control link articulated to a suitable point of said headlamp. In these various systems, the transmission means are of the type employing Bowden cables or levers, or of the hydraulic type.
Reference may be made more specifically to French Patent No. 1,475,011 which relates to a hydraulic device of the type comprising two mechanical transmission systems respectively associated with the front and rear parts of the vehicle and serving to detect movements of the suspended part of the vehicle in relation to the non-suspended part. Each of these transmission systems finally acts on a control lever, the end of which is displaced on either side of a mean position. In this known system, the end of each of the two control levers acts on a hydraulic pickup comprising a variable-volume chamber, the displacement of the lever end resulting in a variation of the volume of said chamber. The hydraulic outlets of the two pickups are interconnected by means of a hydraulic integrator circuit, in relation to which the said pickups are connected in opposition. This prior-art system also comprises a hydraulic control circuit extending from a point of the integrator circuit to a transducer relay acting on a first headlamp, and a connecting circuit connected in series with the control circuit, said transducer relay extending to a control element for the second headlamp. However, none of these prior-art devices can generally be so adapted as to be fitted to motorcycles. The main problem to be overcome, in fact, is due to the difficulty found in providing a front system adapted to detect the relative position of the fork of a motorcycle in relation to its front wheel and transmit such information to means for controlling the vertical orientation of the headlamp. In the great majority of cases, the headlamp is mounted on the fork of a motorcycle and when the latter is stationary the headlamp can be adjusted once and for all to a suitable level and hence orientation of its beam of light is closely linked to the motorcycle attitude. Any variation in the motorcycle attitude therefore has an adverse effect on the quality of the illumination.